Internal combustion engines, in particular diesel and applied-ignition engines, are increasingly being provided with turbochargers. A turbocharger serves for compressing the air supplied to the engine, whereby an increase in performance can be attained. Conversely, a predefined power can be attained by means of a supercharged engine of smaller swept volume, as a result of which it is possible to attain a smaller and more lightweight design and a drive which is more economical in terms of fuel consumption. Such turbochargers are generally driven by the exhaust-gas flow of the internal combustion engine. For this purpose, the turbocharger has a turbine which is arranged in the exhaust-gas flow. The turbine drives, in particular via a common shaft, a compressor which compresses the charge air of the engine.
A greater increase in performance and a higher specific power of the internal combustion engine can be attained through the use of two turbochargers in an internal combustion engine. The two turbines may in this case be arranged in series in the exhaust-gas flow of the internal combustion engine, such that one of the two turbines operates in a region of relatively high pressure and is referred to as a high-pressure turbine, whereas the other turbine operates in the region of relatively low pressure and is referred to as a low-pressure turbine. It is likewise possible for the two compressors to be arranged in series in the charge-air flow. Through the use of a multiplicity of turbochargers in an internal combustion engine, it is also possible to attain an improved throttle pedal response behavior.
A turbocharger arrangement with a multiplicity of turbochargers furthermore offers advantages with regard to exhaust-gas recirculation, since the high-pressure turbine generates a higher exhaust-gas back pressure which permits increased exhaust-gas recirculation; in this way it is possible in particular to attain a reduction in pollutant discharge. Owing to the ever more stringent limit values for the admissible pollutant emissions of motor vehicles, exhaust-gas aftertreatment also plays an ever increasing role. An exhaust-gas aftertreatment system serves in particular to reduce the emissions of nitrogen oxides (NOx), hydrocarbons (HC) and carbon monoxide (CO), and may comprise one or more catalytic converters, for example an oxidation catalytic converter for the reduction of HC and CO, or else LNT (Lean NOx Trap) catalytic converters for the reduction of HC and CO and for the conversion of NOx into N2 and O2. Here, measures may also be taken for reducing the soot content of the exhaust gas. Exhaust-gas aftertreatment systems are generally arranged downstream of the low-pressure exhaust-gas turbine.
For the function of an exhaust-gas aftertreatment system, the inlet temperature, that is to say the temperature of the exhaust gas before or as it enters the exhaust-gas aftertreatment system, should lie within a predetermined range. In particular, for adequate functioning of the exhaust-gas aftertreatment system, the exhaust gases should have a minimum inlet temperature. Since said minimum temperature cannot be ensured at all times, known measures for increasing the inlet temperature include the injection of additional fuel, in particular by means of a post-injection, or throttling the engine. Said measures however lead to an undesired increase in fuel consumption.
The inventors herein have recognized the above issues and provide an approach to at least partly address them. In one example, a method for controlling a turbocharger arrangement of an internal combustion engine, the turbocharger arrangement having at least a first exhaust-gas turbine and a second exhaust-gas turbine arranged downstream of the first, and an exhaust-gas aftertreatment system being arranged downstream of the second exhaust-gas turbine comprises, in a warm-up mode, controlling at least one exhaust-gas turbine so as to increase an inlet temperature of an exhaust-gas flow at the inlet into the exhaust-gas aftertreatment system.
In this way, the exhaust-gas aftertreatment system may be heated by controlling an exhaust gas turbine. In one example, during the warm-up mode, the second exhaust gas turbine may include a bypass valve that is controlled based on exhaust-gas aftertreatment system or engine temperature, while the first exhaust gas turbine may include a bypass valve that is controlled based on engine speed and load, in order to deliver a desired amount of boost pressure. By controlling one turbine bypass valve to provide desired boost pressure and another turbine bypass valve to route heated exhaust directly to the exhaust-gas aftertreatment system, rapid heating of the exhaust-gas aftertreatment system may be achieved without utilizing post-injection or other mechanisms that degrade fuel economy.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.